1. Field of the Invention
The present invention relates to a resin composition constituting the laminate for electric circuit boards and a laminate comprising such a resin composition. More particularly, it relates to a resin composition for reducing the dielectric constant and increasing the solvent resistance of electric circuit boards and a laminate comprising such a resin composition.
2. Description of the Related Arts
To improve the high dielectric constant of the epoxy resin (the Dk is 4.5 at 1 GHz after impregnating a glass cloth), the epoxy resin is mixed with polyphenylene oxide (PPO; Dk is 2.7 at 1 GHz) so that the dielectric constant of the electric circuit boards made thereof can be decreased (Dk is 4.0 at 1 GHz). Although the dielectric constant is decreased by using this method, the solvent resistance and high temperature resistance of electric circuit boards are sacrificed.
Syndiotactic polystyrene (sPS) has the properties of high melting point, low specific gravity, and excellent chemical resistance, which make it very useful in many applications. Since the raw material of sPS, i.e., styrene monomer, is very cheap, and its dielectric constant (Dk is 2.4 at 1 GHz) is similar to polytetrafluoroethylenes resin (PTFE; Dk is 2.2 at 1 GHz), sPS has the potential for developing electric circuit boards. Nowadays, the material used for making high frequency electric circuit substrates is generally PTFE or PPO. See for example, U.S. Pat. Nos. 4,853,423, 5,043,367 and 5,308,565. However, PTFE is relatively expensive ( greater than US$ 20/kg) and difficult in processing; PPO has poor solvent resistance (the solvent resistance is necessary for the cleaning of electric circuit boards). Thus, the present invention aims to replace PTFE or PPO with functionalized-sPS (f-sPS) in order to increase solvent resistance while maintaining the low dielectric constant for making electric circuit substrates.
It is therefore an object of the present invention to provide a resin composition constituting the laminate for an electric circuit board which has high solvent resistance, high temperature resistance and excellent dielectric properties, and which is inexpensive.
Another object of the present invention is to provide a laminate for an electric circuit board comprising such a resin composition.
To attain the above objects, the resin composition in accordance with the present invention is co-polymerized with a functionalized syndiotactic polystyrene-based copolymer which has microfoaming when being cured and a mixture of epoxy resins. The microfoaming of the f-sPS contributes to the decrease of the dielectric constant.
The resin composition according to the present invention comprises: (a) 20-99.9 parts by weight of a functionalized syndiotactic polystyrene-based copolymer having microfoaming when being cured; (b) 0.01-80 parts by weight of a mixture of epoxy resins; and (c) less than 50 parts by weight of a curing agent.
In the resin composition according to the present invention, a functionalized syndiotactic polystyrene-based copolymer having microfoaming when being cured is employed as the component (a). Especially preferred are functionalized syndiotactic styrene/para-alkylstyrene copolymers. Illustrative of such copolymers include oxidized styrene/para-methylstyrene copolymer, halogenated syndiotactic styrene/para-methylstyrene copolymer, carboxylated styrene/para-methylstyrene copolymer, metallized styrene/para-methylstyrene copolymer, aminated styrene/para-methylstyrene copolymer or silylated styrene/para-methylstyrene copolymer. The molecular weight of the functionalized syndiotactic polystyrene-based copolymer to be used in the present invention is preferably within the range of 1xc3x97104 to 5xc3x97105 in terms of weight-average molecular weight.
The general process for preparing the functionalized syndiotactic styrene-based copolymer of the present invention will be described below. Taking the reaction of styrene and para-methylstyrene monomers as an example, the two monomers are co-polymerized by using a metallocene catalyst. The catalyst system may also include an activating cocatalyst such as methyl aluminoxane (MAO). 
wherein x and y are the molar ratio of the respective monomer, and x+y=100.
Suitable metallocene catalysts have a delocalized xcfx84-bonded moiety with a constrained geometry. The catalysts may be further described as a metal coordination complex comprising a IVB-VIB Groups metal and a delocalized xcfx84-bonded moiety with a constrained geometry. In this regard, references are made to U.S. Pat. Nos. 4,542,199; 4,530,914; 4,665,047; 4,752,597; 5,026,798; and 5,272,236. Preferred catalyst complexes include zirconocene and titanocene coordination compounds with single or double cyclopentadienyl derivatives which form the constrained ligand geometry.
The activating co-catalyst can be methyl aluminoxane (MAO), a trialkyl aluminum, a dialkyl aluminum, a salt of an inert and non-coordinating anion, or a mixture thereof. Illustrative of trialkyl aluminum includes trimethyl aluminum, triethyl aluminum, tripropyl aluminum, trisopropyl aluminum, tributyl aluminum, and triisobutyl aluminum (TIBA). The salt of an inert and non-coordinating anion can be borates. Borates suitable for use in the present invention include N,N-dimethyl anilinium tetrakis(pentafluorophenyl)-borate, triphenyl carbenium tetrakis(pentafluorophenyl)-borate, trimethyl ammonium tetrakis(pentafluorophenyl)borate, ferrocenium tetrakis(pentafluorophenyl)borate, dimethyl ferrocenium tetrakis(pentafluorophenyl)borate, and silver tetrakis(pentafluorophenyl)borate. Preferably, the activating co-catalyst is methyl aluminoxane, or a mixture of a trialkyl aluminum and a borate. Suitable diluents for the polymerization reaction include aliphatic and aromatic hydrocarbons such as propane, butane, pentane, cyclopentane, hexane, toluene, heptane, iso-octane, and the like, which can be used individually or collectively.
In general, the polymerization reaction is carried out by mixing styrene and p-methylstyrene in the presence of the catalyst in a co-polymerization reactor, with thorough mixing at a temperature between 0xc2x0 C. to 100xc2x0 C. The polymerization is carried out under an inert gas atmosphere in absence of moisture.
In the styrene/p-methylstyrene copolymer, the benzylic protons in p-methylstyrene unit can be easily converted to various functional groups, such as carboxyl or acid anhydride, amino, hydroxyl or epoxy, under mild reaction conditions. Examples of the carboxyl or acid anhydride include, without limitation, maleic acid, maleic anhydride, acrylic acid, methacrylic acid, citraconic anhydride, itaconic acid, itaconic anhydride, cis-4-cyclohexene-1,2-dicarboxylic acid, cis-4-cyclohexene-1,2-dicarboxylic anhydride or endo-bi-cyclo-2,2,2,1,-5-heptene-2,3-dicarboxylic anhydride. Examples of the epoxy include, without limitation, glycidyl ether, methacrylic acid glycidyl ether, glycidyl methacrylate or acrylic acid glycidyl ether. Most functionalization reactions of benzylic protons in organic compounds can be applied to those of benzylic protons in p-methylstyrene. With regard to the functionallization of benzylic protons, references are made to U.S. Pat. No. 5,543,484 (Chung, et al.); U.S. Pat. No. 5,548,029 (Powers et al.); and U.S. Pat. No. 5,162,445 (Powers, et al.).
In the resin composition according to the present invention, a mixture of epoxy resins is employed as the component (b). The mixture comprises: (b1) 10-90 parts by weight of bisphenol polyglycidyl ether; and (b2) 90-10 parts by weight of epoxidized novolak resin, wherein the epoxy equivalent weight of bisphenol polyglycidyl ether ranges from 160 to 4,000, preferably from 180 to 200; the component (b1) used in the invention has the following structure: 
wherein:
each of A1 and A2 is a monocyclic divalent aromatic radical; and
Y is a substituted hydrocarbon radical used for separating A1 and A2.
The A1 and A2 described above are unsubstituted phenylene or substituted derivatives, wherein the substituent of the derivatives includes, without limitation, alkyl, nitro or alkoxy. Y is a substituted hydrocarbon radical includes, without limitation, methylene, cyclohexylmethylene, ethylene, isopropylidene, neopentylidene, cyclohexylidene or cyclopentadecyidene, wherein the substituent is selected from the group consisting of hydrocarbons, oxy, sulfoxy and sulfone.
In the resin composition according to the present invention, a curing agent is employed as the component (c). The curing agent includes, without limitation, aromatic amine, sec-amine, tert-amine, anhydrous acid or dicyandiamide.
The resin composition according to the present invention can further comprises a curing accelerating agent such as boron trifluoride-amine complex.
The resin composition according to the present invention can also further comprises a filler, such as flame retardants, flame retardant aids, or combinations thereof.
According to another object of the invention, there is also provided a laminate for an electric circuit board, which comprises a sheet made from the resin composition as set forth above being laminated with a glass cloth. The mixture of epoxy resins containing the above-described components (i.e. (b1) and (b2)) is dissolved into a colloidal solution first, and then the f-sPS is dissolved in the colloidal solution. Afterward, a glass cloth is impregnated, and the resulting prepreg is then heated, laminated to form a printed circuit board.
According to the preferred embodiment of the invention, the laminate produced thereby exhibited an excellent dielectric constant of 3.1 at 1 GHz. Due to the similar processing, the traditional raw material, polyphenylene oxide, can be replaced to f-sPS, and the resulting electric circuit board has the advantages of low dielectric constant and increased solvent resistance.
Without intending to limit it in any manner, the present invention will be further illustrated by the following examples.